1. Field of the Invention
This invention relates to a crosslinked nano-inorganic particle/polymer electrolyte membrane suitable for use in a fuel cell, and a method for producing it. The crosslinked nano-inorganic particle/polymer electrolyte membrane of the present invention is excellent in oxidation resistance, heat resistance, and dimensional stability, and has excellent proton conductivity. The present invention also relates to a membrane electrode assembly using the crosslinked nano-inorganic particle/polymer electrolyte membrane.
2. Description of the Related Art
A fuel cell using a polymer electrolyte membrane can be operated at a temperature in the vicinity of 80° C. and, because of its high energy density, is expected to serve as a power source for mobile instruments, home-oriented cogeneration, and automobiles, or as a simplified auxiliary power source, by use of a fuel such as methanol or hydrogen. Development of a polymer electrolyte membrane having excellent characteristics in the fuel cell is one of the most important technologies.
The polymer electrolyte membrane fuel cell is composed of a cell stack consisting of single cells stacked in many layers, each single cell as a power generation unit having a pair of electrode catalyst layers provided on both surfaces of an electrolyte membrane. In this case, the electrolyte membrane acts to conduct protons, and also acts as a diaphragm for preventing direct mixing of hydrogen or methanol as a fuel, and air (oxygen) as an oxidizing agent. The electrolyte membrane is desired to have a great ion exchange capacity; chemical stability of the membrane for long-term passage of an electric current, especially, resistance to hydroxide radicals becoming a main cause of membrane deterioration (i.e., oxidation resistance); constant and high water retention properties of the membrane for keeping electrical resistance low; and excellent heat resistance at the cell operating temperature of 80° C., or at even higher temperatures from the viewpoints of increasing the activity of the electrode catalyst and effective utilization of waste heat. To act as the diaphragm, the electrolyte membrane is required to be excellent in the dynamic strength and dimensional stability of the membrane, and not to have excessive permeability to a hydrogen gas, methanol or an oxygen gas.
The polymer electrolyte membrane fuel cell in the early days used a hydrocarbon-based polymer electrolyte membrane produced by the copolymerization of styrene and divinylbenzene. However, this electrolyte membrane had very poor durability due to low oxidation resistance, and was thus scarcely practical. Thereafter, the perfluorosulfonic acid membrane “Nafion (registered trademark)” developed by DuPont was generally used.
The, conventional fluoropolymer electrolyte membranes, such as Nafion, were excellent in chemical durability and stability. However, their ion exchange capacity was as low as about 1 meq/g, and their water retention properties were so insufficient that the drying of the ion exchange membranes occurred at the low relative humidity, resulting in decreased proton conductivity. They were also disadvantageous in that when methanol was used as a fuel, swelling of the membrane or crossover of methanol took place. If it was attempted to introduce many sulfonic groups into the membrane in order to increase the ion exchange capacity, the strength of the membrane markedly decreased because of swelling, since no crosslinked structure was present in the polymer chains, with the result that the membrane was easily damaged. With the conventional fluoropolymer electrolyte membrane, therefore, it was necessary to limit the content of the sulfonic groups to about 1 meq/g at which the strength of the membrane was maintained.
Furthermore, the monomer for the fluoropolymer electrolyte membrane such as Nafion is difficult and complicated to synthesize, and a process for polymerizing it to produce the polymer membrane is also complicated. Thus, the resulting product is very expensive, constituting a serious impediment to the installation of a proton exchange membrane fuel cell in automobiles for practical use. Efforts have thus been made to develop a low-cost high-performance electrolyte membrane which can replace Nafion, etc.
In the field of radiation graft polymerization which is closely related to the present invention, on the other hand, attempts have been made to graft-polymerize a monomer, which can introduce sulfonic groups, into a polymer membrane, thereby producing a solid polymer electrolyte membrane. The inventors have conducted studies in an attempt to develop a new solid polymer electrolyte membrane, and have obtained the following findings: A styrene monomer is introduced by a radiation graft reaction into a polytetrafluoroethylene film having a crosslinked structure, and is then sulfonated. As a result, the ion exchange capacity can be controlled in a wide range and, since the crosslinks are imparted, the resulting product is minimally swollen with a methanol fuel. The inventors have developed a solid polymer electrolyte membrane characterized by these findings, and a method for producing the membrane (see Japanese Unexamined Patent Publication No. 2001-348439). However, this polymer electrolyte membrane has styrene graft chains composed of hydrocarbons, and thus had the disadvantages that when the fuel cell was operated for a long time, oxidation occurred in some of the graft chain portions, leading to a decline in the ion exchange capacity of the membrane.
The inventors also developed a functional inorganic-graft polymer hybrid ion exchange membrane, and a method for its preparation (see Japanese Unexamined Patent Publication No. 2005-108561). The membrane and the method are characterized in that an inorganic fine powder is mixed with an ethylene-tetrafluoroethylene copolymer, followed by proceeding into a film, to form an inorganic dispersed film, graft polymer chains are introduced into the film, whereafter a crosslinked structure is imparted by radiation and sulfonated. However, the hybrid ion exchange membrane has been found to involve problems, such that the inorganic powder is apt to agglomerate, requiring uniform dispersion and mixing.
The present invention has been accomplished to solve the above-described problems with the earlier technologies. This invention concerns a polymer solid electrolyte which is intended for resolving the drawbacks of polymer ion exchange membranes, including low ion exchange capacity, poor dimensional stability of the membrane, and low oxidation resistance, and the problem, in particular, that the most important inorganic fine powder is liable to agglomeration, thus causing biased presence of the lumpy inorganic material in the membrane, resulting in insufficient strength of the electrolyte membrane produced.
It is an object of the present invention to provide a crosslinked nano-inorganic particle/polymer electrolyte membrane suitable for use in a fuel cell, and excellent in ion exchange capacity, dimensional stability and oxidation resistance, as well as strength, and a method for producing the polymer electrolyte membrane. It is another object of the invention to provide a membrane electrode assembly using such a crosslinked nano-inorganic particle/polymer electrolyte membrane.